A shock testing machine control system is presented which provides improved pulse generation accuracy and repeatability.
The prior art pneumatic shock testing machine consists of a generally rectangular reaction mass assembly that is mounted to shock isolator type feet. The top of the reaction mass assembly may be a meter or so above the floor. A vertically extending cylindrical opening in the top of the reaction mass assembly serves as a receptacle for a charge cylinder assembly which includes an air driven piston. The uppermost end of the charge cylinder assembly terminates at a relatively massive anvil flange which has a multiplicity of bolt holes around its periphery to allow the assembly to be fixedly secured to the topside of the reaction mass assembly. The piston rod of the charge cylinder extends upwardly through the anvil flange terminating at an impact plate. Mounted atop the impact plate is a carriage assembly. Test specimens are secured to the topside of the carriage assembly.
To operate, air under 100 psi pressure or thereabouts is valved to push the piston rod vertically upward to an extended position. There the carriage assembly is stopped and a shock generator element placed on the surface of the anvil. With the shock generator element in place, the cylinder control valves are sequenced to cause the air driven piston to accelerate downward carrying the carriage assembly toward the anvil. This action crushes the shock generator element between the impact plate and the anvil, quickly decelerating the test specimen to zero velocity. The characteristics of the shock generator element determines the shape of the shock pulse experienced by the specimen under test. The height of the piston travel and the amount of air pressure applied to the downward accelerating piston determines the magnitude of the shock pulse. A braking unit within the charge cylinder is used to dampen out carriage rebound after the pulse.
The control system for the prior art machine operates generally as follows. A guide rod approximately a half meter long is secured at one end so as to extend vertically upward from the top of the reaction mass assembly. It is positioned so as to be immediately adjacent to the carriage assembly. Microswitches are mounted on the guide rod in such a manner that the switches are activated by the carriage as it moves along the vertical axis. In order to set the height that the carriage rises above the shock generating device the microswitch that stops the lift cycle is moved up or down the guide rod to respectively increase or decrease the lift height. The carriage will then lift until it activates the switch which sets a pneumatic brake and terminates the lifting action. It is necessary to physically move a microswitch to adjust the lift or fall height parameter. In a like fashion the trigger pulse for a monitor oscilloscope is generated as the carriage activates a microswitch when passing a preset point in the down stroke. This is done by physically moving the switch to the desired trigger point. The application of the pulse dampening brake is also determined by activating a microswitch which is moved to the desired point of brake application by manually moving the switch up or down the fixed guide rod. System timing is derived from a motor driven cam which activates other microswitches to turn the pneumatic valves on and off and thus control the cycling rate of the machine.
My invention improves on the prior art machine by eliminating reliance on microswitches to indicate when an event should occur. I utilize digital logic and optical sensors to determine position. Addition of an on-board microcomputer makes it possible to program parameters which had to be manually set in the prior art machine.